The conversion of solid, low-energy density lignocellulosic biomass into liquid, high-energy density bio-oils that are stable, storable, pumpable and which can be co-processed in near conventional refineries to conventional refinery products (i.e. “drop-in” fuel products) is a primary goal that underpins sustainable biofuel production.
Existing approaches for so-called “first generation” biofuel production commonly use plant seeds leaving the remainder of the plant unused. Apart from being wasteful the biofuels generated by these processes are so-called oxygenates (e.g. ethanol, fatty acid methyl esters) which have a notably lower energy density than fossil diesel or gasoline.
Pyrolysis (heating biomass to very high temperatures in an atmospheric to low oxygen environment) is an alternative approach used to convert biomass into liquid bio-oils. However, liquid bio-oils produced by pyrolysis are generally of a very high oxygen content, resulting in low energy density and increased instability (‘gumming up’) making them difficult to process commercially. Although pyrolysis may be taken further to gasification and the gases utilised in Fischer-Tropsch diesel synthesis to process pyrolysis oils into drop-in liquid fuels, the capital costs involved in doing so are significant which has impeded wide-spread implementation thus far.
Significant progress has been made in the hydrothermal upgrading of pyrolysis oils (both with and without catalysts) to produce more stable oil products with lowered oxygen content. However, these processes still suffer from difficulties that significantly impact on upscaling/commercial operation including, for example, constraints on the proportion of feedstock in reaction slurries, sub-optimal heat transfer, and product separation.